Python tkinter label image not drawn unless a message widget is behind it
I suspect it has to do with the fact that a tkinter window by default is 1x1 pixel in size. When you use place, tkinter will not expand the window to fit its contents.
The reason it works with the messagebox is because you are adding the messagebox to the root window with pack which will cause the window to grow.
The simple solution is to use new_plot.pack(...) instead of new_plot.place(...).